fix mpp_error output bug

[rem1776]

Description
Fixes a un-initialized variable for GCC 15.

This bug was causing any strings output by mpp_error to be garbage non-ascii values, which is particularly annoying.

Fixes #1858

How Has This Been Tested?
make check with gcc 15 on amd box

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• make distcheck passes

[uramirez8707]

Thanks @rem1776